The crucial roles of transition metals in fundamental biological processes underscore the importance of a sound theoretical understanding of bonding interactions in multi-metal compounds. Regrettably, the development of theoretical models capable of describing the elaborate metal coordination units of biomolecules is hindered by the lack of adequate spectroscopic information concerning far simpler transition metal compounds. The principle objectives of the research described in this proposal are to obtain accurate spectroscopic information concerning the electronic and molecular structures of several of the simplest transition metal cluster compounds, and to compare these experimental results with the predictions of molecular orbital calculations employing the extended multiple-scattering XAlpha method. Attention will focus on clusters of iron or molybdenum bonded directly or via bridging oxygen or sulfur atoms, in order to gain insight into the properties of the more elaborate cluster sites of similar composition found in a variety of metallobiomolecules. These include the hemerythrins, whose binuclear iron sites function in dioxygen binding and transport, the ferredoxins, whose 2Fe-2S and 4Fe-4S coordination units play central roles in electron-transfer processes, and the nitrogenase enzyme complex, whose Fe/Mo/S groups may provide the catalytic sites for the reduction of dinitrogen to ammonia.